1. Field of the Invention:
The present invention relates in particular to hemodialysis: It is an apparatus for measuring an amount of ultrafiltrate produced in the hemodialysis and a concentration of the receiving solvent employed in the hemodialysis by the use of ultrasonic waves.
2. Description of the Prior Art:
In recent years, in the field of hemodialysis, the performance of hemodialyzers is remarkably improved. As a result, there is a strong demand for methods that can measure the amount of ultrafiltrate or water removed from blood after having been dialyzated. In the majority of cases, the flow rate of the receiving solvent employed in the hemodialyzer reaches an amount of 500 ml per minutes while that of the ultrafiltrate produced in the hemodialyzer reaches up to only several percent of 500 ml, which reaches that the amount of the ultrafiltrate is measured with accuracies of up to 0.1% of 500 ml.
The conventional methods for measuring the amount of ultrafiltrate produced in the hemodialyzer can be classified into three categories. All of these conventional methods, however, suffer from its inherent defects as follows. In the first method, a pair of fixed-volume chambers is employed, each of which is separated into two rooms by a movable partition. Through this input-flo rate to the hemodialyzer is kept equal to an output-flow rate from the same by the use of a selector valve, while ultrafiltration is forcibly conducted by the use of other means. Although this method is excellent in accuracy, it is too complex and can be conducted only by the use of an apparatus provided with many expensive components. In addition, the selector valve tends to be worn out quickly because of excessive operation, which deteriorates the accuracy of the apparatus. However, it is hard to detect the deterioration of the accuracy. Next, in the second method, both an input circuit and an output circuit of the hemodialyzer are temporarily shut off in order that the pressure across a membrane and the amount of ultrafiltrate are determined, whereby the amount of ultrafiltrate produced at a time when these input and output circuits are open is calculated on the basis of the pressure across the membrane and the relationship between the above determined values. Although the second method is simple in construction to make it possible to determine the amount of ultrafiltrate, its accuracy is reduced when it is conducted by the use of a high-performance hemodialyzer or a specialized hemodialyzer. In addition, since the conditions of the moment when the circuits are shut off differ from those of the moment when the calculation is conducted and change at each hour of operation, the thus determined values are poor in reliability. Lastly, in the third method, both an input and an output flow rates in the hemodializer are directly determined by means of a flow meter to find differences in flow rate therebetween, so that the amount of ultrafiltrate is determined on the basis of such differences in flow rate. Although this third method is basic, it is seldom employed in practical use, because it is hard to obtain a flow meter with a sufficient accuracy in use and an outlet side of the hemodializer is seriously soiled with dirt. In this connection, Japanese Patent Publication No. 59-10227 discloses an apparatus provided with a flow meter which is poor in accuracy and is changed over in use to make it possible that its inlet circuit also serves as an outlet circuit thereof, and vice versa. Such change-over use of the circuits seriously contaminates the apparatus, which in turn makes it necessary to disinfect the apparatus frequently.
On the other hand, in determining a concentration of the receiving solvent employed in the hemodialyzer, the electrical conductivity of the receiving solvent is generally determined by means of metallic or carbon electrodes. In recent years, there is an incessant growth in demand for solutions of bicarbonates which might serve as the receiving solvents in the hemodialyzer, because the solutions of bicarbonates do not adversely affect the patient substantially. The receiving solvent of this type has a problem in that it tends to produce carbonates deposited on the electrodes when the composition of the receiving solvent changes, the carbonates being an electrically insulating materials. Consequently, it is hard to control the concentration of the receiving solvent of this type, which leads to difficulty in giving the alarm for an abnormal concentration of the receiving solvent. Such abnormal concentrations of the receiving solvent in use are very dangerous to for the patient.